Project III is designed to discover mechanisms that underlie the ability of the immature cardiomyocyte to integrate conflicting hormonal signals that determine the final and optimal cardiomyocyte endowment at birth. Fetal cardiomyocyte endowment is important because it determines the relative force that must be generated by each working cell for the life of offspring and it guides the generation of the coronary architecture within the myocardium. In rats, underendowed hearts are more vulnerable to ischemia-reperfusion injury and heart failure. Based on PPG support we have demonstrated that three hormone/growth factors stimulate cardiomyocyte growth in living near term fetuses: insulin-like growth factor-1 (IGF-1), angiotensin II (Ang II) and Cortisol. Two factors inhibit stimulated cardiomyocyte growth: atrial natriuretic peptide and tri-iodo-L-thyronine (T3). For this project we will study the signaling and actions of IGF-1, T3 and Cortisol. We propose 3 aims that will further our understanding of how these hormones work together and how they might be used to augment cardiomyocte numbers in under-endowed growth restricted hearts. Aim 1 (in vitro) will test the hypothesis that the hierarchy of signaling action from IGF-1, Cortisol and T3 is based on signal interactions within the mitogen activated protein kinase (MAPK), extracellular regulatory kinase (ERK) and the phosphoinositol 3 kinase (PI3K) pathways in the near term fetal ovine cardiomyocyte. Aim 2 will test three hypotheses to determine the roles of IGF-1, Taand Cortisol in regulating cardiomyocyte growth and maturation. The aim speculates that phosphorylation of ERK and PI3-K pathway proteins will be down-regulated in cardiomyocytes in response to subnormal T3 levels which will lead to decreased IGF-1 receptors and increased IGF-1 concentrations. Aim 3 (in vivo) will test the hypothesis that exogenous IGF-1 and T3 can be used to augment cardiomyocyte numbers permanently in the fetal myocardium. The outcome of the project will be possible therapies for repairing disadvantaged hearts in the womb.